Add methods for efficient counting of paths between vertices.

[sbeitzel]

This adds several methods to the Graph protocol to allow some optimizations for counting the number of paths between two vertices.

[Copilot]

Pull request overview

This pull request adds several methods to the Graph protocol to enable efficient counting of paths between vertices in a graph. The implementation includes optimizations using reachability precomputation to avoid exploring dead-end paths.

Key Changes:

• Added methods for path counting with exhaustive search algorithms including basic and optimized versions
• Added helper methods for checking path existence and computing reachability
• Added comprehensive test coverage using an Advent of Code sample graph

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 10 comments.

File	Description
Sources/SwiftGraph/Graph.swift	Adds five new methods to the Graph extension: neighborIndicesForIndex, pathExists, two overloads of countPaths (basic and optimized with reachability), and reachabilityOf for computing vertex reachability
Tests/SwiftGraphTests/SwiftGraphTests.swift	Adds Foundation import, new test method testExhaustivePathCounting to exercise the path counting methods, and helper method getAoCGraph that deserializes a test graph from JSON

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[davecom]

Thanks for the contribution, @sbeitzel